Various requirements have arisen for compact chemical reactors to be used in a variety of industrial and commercial settings. For instance, the need is rapidly arising to economically produce small amounts of hydrogen for the potential hydrogen fuel cell economy. For such purposes, it is desirable that hydrogen be produced by known catalytic reactions such as steam methane reforming followed by water-gas shift and that the reactor be as physically small as possible. The requirement that the reactor be compact is necessary to allow such reactor to be situated at existing automotive fuel stations where space is typically at a premium.
In the prior art, a variety of compact catalytic reactor designs have been proposed. For instance, U.S. Pat. No. 4,737,161 discloses a compact hydrogen generator in which a helical tube serving as the reaction zone is situated within a housing having an axial burner. Another similar device is disclosed in U.S. Pat. No. 3,357,916. In this patent a chemical reactor is disclosed. In one embodiment, a reactor shell or housing contains a length of helical tubing to serve as a reaction zone. The helical tube has a catalyst embedded on its inner surface to promote chemical reactions. A hydrocarbon feed can be passed through the helical tubing to be cracked in the presence of the embedded catalyst. Part of the cracked feed can be combusted within the reactor shell to generate heat to support the endothermic cracking reaction.
Other compact reactors utilize a series of concentric shells in which coiled tubes are used for heat exchange. For instance in U.S. Pat. No. 6,254,839 a reformer vessel is provided having a partial oxidation zone that is located beneath the steam reforming zone and that contains a steam reforming catalyst. The two zones are centrally and axially located within the reformer vessel. The partial oxidation zone and the catalytic reaction zone are surrounded by a helical tube and an oxygen containing source stream, or alternatively, fuel and steam, can be introduced into the helical tube to be preheated. Surrounding the helical tube is a shift region containing shift catalyst. A cooling zone, which can be helical tubing, is located within the shift region to receive cooling water to provide lower reaction temperatures that favor the shift reaction.
The reactor designs discussed above provide a compact arrangement for the catalytic reaction of the feed. However, the coiled tube design, while permitting the fabrication of a compact reactor, at the same time, is in and of itself a design limitation on the unit operation or operations to be conducted within the reactor. In this regard, the length of the tube limits the amount of reaction taking place within the reactor. As tube length is increased, the pressure drop within the tube also increases and therefore, energy must be expended at the feed end to overcome such irreversible loss. Additionally, as the spiral is made longer to accommodate a longer length of tubing, even heat transfer and thermal insulation can become problematical because of the greater length over which heat transfer takes place and/or thermal insulation is provided.
As will be discussed, the present invention provides a compact reactor that utilizes a helical tubular type configuration for reaction purposes and preferably, also, for heat transfer purposes but is superior to the prior art discussed above with respect to both its pressure drop and heat exchange characteristics.